In the disk recording art, it is common to use read/write heads which "fly" over the surface of the rotating disk. Aerodynamic surfaces on the head assembly react against the air moved with the revolving disk causing the heads to "fly" a small distance from the disk surface. In the manufacture of such read/write heads, it is common to test the aerodynamic characteristics of the head so that the flying height characteristics are known, thereby avoiding use of heads which "fly" too high or too low in relationship to the disk surface or at an improper angle to the disk.
Flying height testing is ordinarily accomplished by means of a flying height tester, using optical interference technique. Such a flying height tester comprises, for example, a monochromatic light source such as a tunable monochroaator directing monochromatic light at a glass disk. The glass disk is rotated at speeds simulating the rotation of a magnetic disk, and the head/arm assembly being tested is positioned in a holder or clamp in flying relation to the revolving glass disk. Monochromatic light is directed at the disk at a predetermined angle to the surface thereof. Light is reflected from the surface of the disk closest to the flying head, and from the surface of the flying head itself, and impinges onto a sensor, such as a television camera or other light-sensitive sensor. With the head in its flying position, monochromatic light reflected from the disk and from the head impinge upon the sensor in additive (constructive interference) and subtractive (destructive interference) modes to create a pattern of light sectors of bright and dark sectors, the sectors being bounded by "fringes" as they are known in the industry. The distance between the fringes is representative of the angle between the head and the disk, and the position of the fringes relative to the image of the edge of the head is representative of the distance, or flying height, between the head and disk. A computer, such as a Hewlett-Packard HP-85 computer, programmed to receive data from the head tester, can calculate the perceived flying height and angle of the head using the detected fringe pattern distances. For further details concerning flying height testers using optical interference techniques, see "Techniques for the Measurement of Air-Bearing Separation--A Review" by C. Lin, IEEE Transactions on Magnetics, Vol. MAG-9, No. 4, pages 673-677 (December 1973).
In the past, calibration of flying height testers has been accomplished through the use of a standard head whose characteristics are known. However, arm flight characteristics, gimble spring characteristics, etc., of the standard head change after repeated use and abuse. Further, dust, skin oil and other foreign matter may alter the flying characteristics and/or reflective surface of the standard head, altering the calibration of the standard. Because the flying orientation of a head is in part affected by the speed, radius, and stacking height of the magnetic disk, the actual field conditions of a head could not be duplicated and it was necessary that heads be accepted that have characteristics within given acceptable ranges. Calibration of production heads could only be accomplished in relationship to the calibration of the tester by the given standard; the standard could not be calibrated in absolute terms. It was not possible to calibrate heads on different machines with any degree of relative accuracy.